This invention relates to a method for preventing or arresting the development and propagation of fatigue cracks in the material surrounding a fastener without removing and reinserting the fastener. The invention is particularly applicable to metallic materials where stress related failures, such as fatigue and stress corrosion cracking (scc), are a primary concern. Such materials would include, though not exclusively, those materials used in aerospace applications where stress related failures have potentially catastrophic consequences.
Interruptions or discontinuities, such as fastener holes, in an otherwise continuous material serve as stress risers when the material is placed under load. This means that the material immediately surrounding the discontinuity experiences greater stress than the balance of the material even though the applied load may be uniform over the entire part. The material surrounding these discontinuities is, therefore, more susceptible to crack initiation and ultimately failure. This effect is especially pronounced when the component undergoes cyclical loading and unloading as the part ultimately fails due to fatigue. Failures may also occur as a result of stress corrosion cracking in such areas when the component is exposed to corrosive environments such as salt water. Cracks may also initiate from flaws in the material such as corrosion pits, fretting damage and similar flaws.
Materials used in aircraft applications are especially susceptible to such failures, particularly in airframe structural members and aircraft skin materials. More specifically, fatigue cracks often develop in the material surrounding the multitude of fastener holes in the fuselage of an aircraft. As the fuselage is placed under load, such as when the fuselage is pressurized during operation of the aircraft, the material surrounding these holes experiences a greater amount of stress than material elsewhere in the structure. Over many cycles of loading and unloading such high stresses contribute to the fatigue failure of the part from cracks initiating in the material surrounding the hole. The development of such cracks may have potentially catastrophic effects. The possibility of such failures is ever increasing as aircraft in commercial and military fleets age.
Prior attempts to solve this problem involve inducing compressive residual stresses in the material surrounding the fastener hole. A variety of techniques for inducing compressive residual stresses have been previously used including stress coining, indenting, split-sleeve cold expansion, split mandrel cold working, and ballizing. U.S. Pat. No. 6,711,928 —Easterbrook (patented Mar. 30, 2004)—includes an overview of these various methods for improving the fatigue performance of a hole or slot. While these techniques are well suited for forming a pre-stressed hole or slot in which a fastener may be inserted, they are not well suited for the in situ treatment of the material surrounding fasteners as they would first require the removal of the fastener. For structures such as an aircraft fuselage skin, this would require the removal and replacement of thousands of fasteners which would be extremely laborious, time consuming and expensive. Therefore, such methodologies are not well suited for the repair of existing structures.
Further, the residual stress distribution introduced by stress coining and indenting cannot be precisely controlled to maximize the benefit of the induced compressive residual stress distribution. Also, the coining and indenting processes treat the entire area surrounding the fastener hole in a single operation. This requires a great deal of force to obtain the desired compressive residual stresses. The application of such extreme forces could potentially damage the structure or skin of the aircraft.
Therefore, the need exists for a cost effective and efficient means of introducing residual compressive stresses in the material surrounding a fastener hole or slot without removing the fastener to prevent or arrest the development and propagation of cracks.